Field of the Invention
The present invention relates to a novel titania photocatalyst and its manufacturing method. More specifically, the present invention is to provide the quantum-sized novel titania photocatalyst prepared the steps comprising: (a) titanium tetraisopropoxide is encapsulated in zeolite support by adding citric acid to isopropyl alcohol; (b) ethylene glycol is dissolved herein to obtain a uniformly dispersed mixture solution; and (c) it is encapsulated in zeolite cavities. And thus, titania photocatalyst of the present invention has some advantages in that (a) it provides greatly increased surface area and photocatalytic activity due to the smaller granule than the commercial titania powder; (b) it is uniformly dispersed to quantum size in zeolite cavities rather than forming large clusters caused by the aggregation of the conventional titania hyperfine powder; and (c) since the quantum efficiency of titania powder in the UV region is maximized thereby, it effectively and promptly removes the hazardous gas like ammonia and sulfide in the atmosphere and organic material in water waste through photo-oxidation reaction.
Photocatalysts have strong oxidation and reduction activities by UV light contained in the sunlight or fluorescence lamp. Among the photocatalyst, titania (TiO2) is widely used due to its advantages such as excellent photocatalytic activity, photo-stability, chemical and biological stability, durability and low cost. Titania compound has an anatase, rutile or brookite structure of which the anatase structure has the highest photo catalytic activity. When a titania is excited by light absorption with energy equal to or greater than the band gap thereof, it generates electron-hole pairs. The electrons and holes give strong photo-reducion activity and photo-oxidation activity, respectively. The band gap of titania is about 3.2 eV, thus the illumination of the light with wavelength shorter than 380 nm induces the reaction. The characteristic of titania is that the oxidative activity of the holes is much stronger than the reductive activity of the excited electrons. Potential energy of the holes is about +3V by the hydrogen reference voltage, which is much stronger than 1.36V of chlorine or 2.07V of ozone used in the waste water treatment.
Titania can sterilize, decompose, decontaminate and purifiy the noxious materials attached on the surface, in the atmosphere, or in the waste water through the photo-catalytic reaction. So, it is suitable for various uses such as cooler filter, glass, tile, external wall, food, plant inner wall, metal product, aquarium, purification of ocean pollution, dry material, mold prevention, UV blocking, purification of water, purification of air, decontamination in hospital, etc. If this photocatalyst is put on the material surface, dirt or odorous components are naturally decomposed by light. If titania photocatalyst is mixed with concrete or paint, it oxidizes nitrogen oxides (NOx) or sulfur oxides (SOx) which are the cause of acid rain to acetate ions. Namely, the air pollutant is removed only by laying blocks containing photocatalyst on the road or applying paints containing photocatalyst on the buildings. Also, the organic materials like acetaldehyde in the air, which is the cause of bad smell, can be decomposed. If titania covered with porous silica gel is mixed with waste liquids containing hardly-decomposed colored pigment, they become colorless under UV irradiation. Also, if the surface where the pollutant is adhering is covered with transparent photocatalyst, the pollutant is degraded naturally. Besides, a lot of researches have been carried to use the photocatalyst for the ordinary goods such as tiles that decomposed pollutant naturally and has anti-bacterial activity, paper that decomposes the smell of cigarettes and mirror or glass that is not affected by moisture.
However, since the conventional photocatalyst has low quantum efficiency, defined by the number of reaction product in unit hour divided by the number of illuminated photons, with only 0.4-8% per unit photocatalyst, the reaction rate is slow. Namely, it cannot quickly decompose hazardous gas components like ammonia, nitride, sulfide, aldehyde, volatile organic compounds and chlorinated volatile organic compounds in the atmosphere or organic compounds in the waste water.
Titania powder has generally 30 nm of an average particle size and 50 m2/g of surface area. If the particle size of the photocatalyst is reduced, its reactivity increases due to the quantum efficiency and the specific surface area increases. So, the use of metal alkoxide to obtain titania powder with small particle size and well-controlled aggregation state was disclosed [Japanese Patent Heisei 96-338671; Langmuir, vol. 1, 414, (1986)]. However, though this method partly decreases the crystal size and the surface area and photoreactivity increase about 2 times, the particle size tends to be larger than the titania product (Degussa""s P-25) because of the aggregation of small particles.
Dohyeong Kim, et al. of POSCO provides a method of preparing titania powder via titanium hydroxide using titanium metal salt instead of metal alkoxide [Korean Patent Publication No. 2000-0039147]. Although this method also partly decreases the crystal size to 10-50 nm, the small particles coagulate to 200-1000 nm of secondary particles and the increase of the specific surface area is limited to 50-120 m2/g.
Also, a preparing method of spherical anatase titania powder with 20 nm of size in the mixture of alcohols, titanium alkoxide and acetic acid at 273xc2x0 C. of high temperature and 7.3 MPa of high pressure for 2 hr in the supercritical fluid of alcohols is disclosed [Korean Patent Publication No. 1999-0054058]. This method also uses very complicated preparing process and reaction equipment. And, though the spherical anatase titania powder is prepared by consuming energy of high temperature and high pressure, the small particles coaggregate to large secondary particles.
In practical sense, general photocatalysts are divided into two groups: i.e., using titania in powder phase and forming thin film on a specific support through the sol-gel method. For the sol-gel method, Sangbeom Han of HanKook Jungsoo Industries, Co., Ltd. developed a preparing method of anatase structure titania powder by forming a photocatalytic source sol after dissolving in some solvent, coating the photocatalytic source sol in the form of thin film or membrane on glass plate, glass bead, porous glass bead, fiber net, honeycomb, or ceramic carrier like ceramic tile and ceramic plate, and then baking the coated support to gel form in the air [Korean Patent Publication No. 1998-035033]. And, Hyeongho Kim provided a method of applying and spraying porous inorganic adhesive and mineral on the thin film support, and fixing the titania powder by pressing it [Korean Patent Publication No. 2000-0058790].
Anpo et al. of Osaka Prefecture University in Japan and Sang-Eon Park et al. of Korea Research Institute of Chemical Technology jointly announced that the dispersion of titania particle affect on the reaction activity. It is dispersing titanium ammonium oxalate aqueous solution on Y-zeolite with specified size via both ion exchange method and impregnation method, and then applying this to photo-catalytic reduction of carbon dioxide [Stud. Surf. Sci. Catal., vol. 114, 177, (1998)]. The result shows that titania particle prepared by ion exchange method provides superior photo-reduction activity than the one prepared by impregnation method. It was observed that if the concentration of titanium ammonium oxalate is increased by the impregnation method, the titania particles co-aggregate and therefore the photo-reduction activity worsens. Using the method reported by Anderson et al. [J. Membrane Sci., vol. 39, 243 (1988)], Langford et al. (University Technologies International of Canada) provided the preparing method of titania powder, wherein anatase structure, rutile structure and brookite structure were mixed, by preparing titania sol in ethanol solution of titanium tetraisopropoxide, supporting it on the support such as ZSM-5, zeolite-A, alumina and silica [U.S. Pat. No. 5,981,426].
However, the titania photocatalyst prepared by the said sol-gel method does not provide sufficient photo-reaction activity to be used industrially.
The inventors tried to prepare a titania photocatalyst with minimum particle size that maximizes the quantum efficiency under UV irradiation and prevent from co-aggregating. As a result, the present invention was completed by inserting citric acid in isopropyl alcohol solution of titanium tetraisopropoxide, adding ethylene glycol in mild acidic condition to obtain a mixture solution wherein the titania particles are uniformly dispersed, and then encapsulating the same in cavities of various zeolite supports. The titania powder prepared in this method is uniformly dispersed with small size compared with the conventional titania powder, and because the formed titania particles reside inside the cavities of the zeolite carrier, they never co-aggregate above a certain size. So, hazardous materials and organic materials in the atmosphere and waste water can be removed quickly in the UV region with dozens of quantum efficiency than the conventional method. Its absorption spectrum was observed to shift to short wavelength region by 20 nm-80 nm.
Accordingly, an object of the present invention is to provide an environment-friendly novel photocatalyst with superior photo-oxidation activity which can be utilized for the purification of atmosphere and waste water, by supporting titanium tetraisopropoxide on various zeolite carriers via sol-gel method in mild acidic condition.